1. Field of the Invention
The present invention relates to an electronic ballast for maintaining the lighting of a gas discharge lamp, and more particularly to an electronic ballast for maintaining the on-status of a high-voltage gas discharge lamp by using an analogically-configured circuit.
2. Description of the Prior Art
Generally, when maintaining the lighting of a gas discharge lamp, especially metal halide lamp, by means of an electronic ballast, flame fluctuates unstably or acoustic resonance is produced. A method for stably maintaining the lighting of the gas discharge lamp while preventing the unstable flame fluctuation or acoustic resonance is disclosed by U.S. Pat. No. 6,107,754. In this method, the light-ON state of the gas discharge lamp is maintained in a manner that the frequencies of driving signals required for lighting the gas discharge lamp is varied within a preset bandwidth in a stepped level to be periodically repeated for the purpose of preventing the flame fluctuation or acoustic resonance during the light-ON state. That is, the method employs a digital controlling means such as a microprocessor to provide the driving frequencies and waveforms to an inverter circuit.
Meanwhile, if the metal halide lamp is driven by a high frequency, the frequency is commonly over 100 Khz. For forming such a waveform by means of the microprocessor, the microprocessor employed should have a speed of 2 Mips or higher when processing an instruction. In addition to incurring high cost, such a high performance microprocessor has a drawback of being vulnerable to external noises because of the increased processing speed. Furthermore, in case of a high voltage gas discharge lamp, a voltage as high as several to several tens of kilovolts is required for initiating the lighting of the lamp. At this time, the signal liably introduced as a noise to the microprocessor which is operated by a voltage of 5volts induces a malfunction of the microprocessor or occasionally turns off the lighted gas discharge lamp, while in severe cases, it fatally damages the ballast.
In view of the above, a first object of the present invention is to provide an electronic ballast capable of stably maintaining the light-on operation of a gas discharge lamp in such that, instead of employing a microprocessor which is a digital control means, periodical repeating of shifting driving signals required for an inverter circuit for supplying an electric power source to the gas discharge lamp from f1 to f2 is embodied by an analog system.
A second object of the present invention is to provide an electronic ballast for a gas discharge lamp capable of stably maintaining the output of the gas discharge lamp by means of a frequency control.
A third object of the present invention is to provide an electronic ballast for a gas discharge lamp capable of performing a safe starting by appropriately adjusting the initial starting output.
A fourth object of the present invention is to provide an electronic ballast for a gas discharge lamp having a relatively large output practical by optionally controlling a dead time when driving an inverter.
The last object of the present invention is to provide an electronic ballast for a gas discharge lamp capable of being protected against over-current, over-heat, no load and inferiority of the gas discharge lamp.
To achieve the above objects, the present invention provides an electronic ballast for a gas discharge lamp which comprises a power source section for receiving and transforming an AC power source to a DC voltage, and an inverter section which, in response to switching driving signals, switches an output voltage of the power source section at a high speed as an AC power source and provides the AC power source to a gas discharge lamp.
In the electronic ballast, an inverter control section generates a pair of switching driving signals respectively having a predetermined dead time within a switching-ON time interval and a phase difference of 180 degrees from each other, while an auxiliary power source section, by means of the output voltage of the power source section, supplies an auxiliary power source required in the inverter controlling section, and a starting section generates a high voltage when performing a starting operation.
Here, the inverter controlling section has an inverter driving unit controlled by a predetermined frequency shift signal for supplying the switching driving signals to the inverter section by being sequentially shifted from a reference frequency to another predetermined frequency for one period, and further has a frequency shift unit for supplying the predetermined frequency shift signal to the inverter driving unit. At this time, the inverter driving unit includes a PWM IC, and an output dividing circuit for dividing two output signals of the PWM IC to provide them as the switching driving signals. The frequency shift unit has an analog circuit for gradually changing a resistance value applied to an RT terminal for determining the output signal frequency of the PWM IC in accordance with a time for one period and then periodically repeat the change of the resistance value. According to the aforementioned configuration, the inverter control circuit gradually shifts the frequency of the switching driving signals that drive the inverter circuit from an optional frequency f1 to another frequency f2 in accordance with the time, and the control of periodically repeating the above-stated frequency shift is embodied by the analog system. Thus, the flame fluctuation or acoustic resonance which occurs during the maintaining of the light-on operation of the gas discharge lamp is prevented to provide the stable lighting operation.
Preferably, the inverter controlling section further has a constant output control unit for stabilizing an output of the gas discharge lamp in a rated output level by the feedback of the power source applied to the gas discharge lamp.
More preferably, the inverter controlling section includes a starting control unit for controlling the frequency of the switching driving signals to be lower than a constant output frequency during the starting of the gas discharge lamp to heighten the output of the gas discharge lamp, thereby allowing the gas discharge lamp to be easily started.
Furthermore, it is preferable that the inverter controlling section includes a protection circuit unit for protecting the ballast by blocking the switching driving signals in case of overheat of the ballast, starting failure and over-current.